Jason D. Arroyo1, John R. Chevillet2, Evan M. Kroh2, Ingrid K. Ruf2, Kevin Lin3, Donald E. Gibson3, Patrick S. Mitchell2, Christopher F. Bennett2,4, Era L. Pogosova‐Agadjanyan5, Derek L. Stirewalt5, Jonathan F. Tait3, Muneesh Tewari2,6,7
1Division of Human Biology, Fred Hutchinson Cancer Research Center, Seattle, WA 98109-1024 USA.
2Division of Human Biology, Fred Hutchinson Cancer Research Center, Seattle, WA 98109-1024;
3Department of Laboratory Medicine, University of Washington Medical Center, Seattle, WA 98195-7110;
4Molecular and Cellular Biology Program, University of Washington, Seattle, WA 98195; and
5Fred Hutchinson Cancer Research Center
6Divisions of dClinical Research and
7Public Health Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA 98109-1024
Tóm tắt
MicroRNAs (miRNAs) circulate in the bloodstream in a highly stable, extracellular form and are being developed as blood-based biomarkers for cancer and other diseases. However, the mechanism underlying their remarkable stability in the RNase-rich environment of blood is not well understood. The current model in the literature posits that circulating miRNAs are protected by encapsulation in membrane-bound vesicles such as exosomes, but this has not been systematically studied. We used differential centrifugation and size-exclusion chromatography as orthogonal approaches to characterize circulating miRNA complexes in human plasma and serum. We found, surprisingly, that the majority of circulating miRNAs cofractionated with protein complexes rather than with vesicles. miRNAs were also sensitive to protease treatment of plasma, indicating that protein complexes protect circulating miRNAs from plasma RNases. Further characterization revealed that Argonaute2 (Ago2), the key effector protein of miRNA-mediated silencing, was present in human plasma and eluted with plasma miRNAs in size-exclusion chromatography. Furthermore, immunoprecipitation of Ago2 from plasma readily recovered non–vesicle-associated plasma miRNAs. The majority of miRNAs studied copurified with the Ago2 ribonucleoprotein complex, but a minority of specific miRNAs associated predominantly with vesicles. Our results reveal two populations of circulating miRNAs and suggest that circulating Ago2 complexes are a mechanism responsible for the stability of plasma miRNAs. Our study has important implications for the development of biomarker approaches based on capture and analysis of circulating miRNAs. In addition, identification of extracellular Ago2–miRNA complexes in plasma raises the possibility that cells release a functional miRNA-induced silencing complex into the circulation.
